1. Field of the Invention
This invention relates to a method for detection of a coating material on a substrate and more particularly to a method for detecting and/or distinguishing the presence of an organic solderability preservative on a copper substrate.
2. Description of the Related Art
Bare copper surfaces oxidize on exposure to air and develop a patina containing copper hydroxide and various other copper compounds. Even invisibly thin coatings of this patina are sufficient to interfere with the ability of solder to wet and form a bond with the copper surface. Surface oxidation renders a copper substrate non-solderable, and unusable. Hence, newly etched copper used in electronic circuit boards is coated with a solderability preservative.
Use of imidazole as an organic solderability preservative ("hereinafter, "OSP") is described in U.S. Pat. No. 4,373,656, herein incorporated by reference. Other OSP's are also known and include various azole derivatives such as benzimidazole, benzotriazole, and alkyl imidazoles. OSP's generally work by forming a complex with copper, which prevents further surface oxidation without diminishing the solderability of the surface.
Detection of the OSP should be performed immediately after the coating process as a process control step, or in the field prior to the soldering process. One problem arises from the fact that the OSP coating is present in very low quantities. Very often, the OSP coating has a thickness of only one monomolecular layer of OSP-copper complex.
A large percentage of the printed circuit boards currently being used are coated with imidazole. Imidazole deposition is a delicate process which would be dramatically improved with a detection technique used as a process monitoring tool. Such a technique should meet three criteria: it should be non-destructive, it should be sensitive enough to detect minute levels of the OSP, and it should be simple to perform with minimal training of the personnel performing the inspection procedure.
Various attempts have been made to achieve such a method: FT-IR reflective techniques, use of a quartz crystal oscillator microbalance, fluorimetry, viscometry, spectroscopy, ESR techniques, XPS, IR, and electrochemical methods such as voltammetry, tensammetry, and SERS. Chemically, imidazole has been detected by its reaction with epoxy compounds. Absence of OSP has also been detected by nitric acid and silver nitrate drop tests.
Many of these methods do not meet the above criteria for a non-destructive, sensitive method which is easily learned by new personnel. Some of the methods require years of advanced study to properly perform the tests and accurately interpret the results with acceptable efficiency. Also, some of the methods mentioned above are for use in aqueous solutions which require a significant area of sample preparation relative to the surface area of the printed wiring product.
We have discovered an improved technique for detecting an OSP coating especially imidazole on copper. The method is non-destructive, sensitive enough to detect monomolecular layers of coating, and employs relatively simple instrumentation with which a high level of operator skill can be acquired in a matter of hours. The technique is both simple and accurate and may be broadly applied to various types of coating materials and substrates.